Introduce "dynamic prompt" to show context-dependent prompts

[emmanuel099]

This allows to print context-dependent prompts based on the current state of the data object.

My use-case:

Loading 'a.o'
[] > ls
...
__libc_csu_init@00000008
_start@00000009
main@0000000A

[] > sel __libc_csu_init
[__libc_csu_init] > cfg
[__libc_csu_init] > dis
[__libc_csu_init] > sel main
[main] >

The sel command allows to "select" a function and perform commands based on the currently selected function. Immediately seeing which function is currently selected is quite useful, especially when performing many commands.